1. Field of the Invention
The present invention relates to a system for attenuating noise electronically and, in particular, to an electronic noise attenuation system which is capable of attenuating non-steady noise occurring in propagation passages such as duct lines or the like by exercising an adaptive control using a computer system including a digital filter therein.
2. Description of the Prior Art
Conventionally, there has been widely put into practical use a passive noise attenuation apparatus which attenuates noise occurring within ducts by use of the interference due to the duct structure or the noise absorption due to a porous material attached to the duct. However, this type of noise attenuation apparatus is found disadvantageous in that it is too big in size, it involves too much loss of pressure, and so on.
On the other hand, there is also available an active noise attenuation apparatus which has been long proposed and employs another method of the reduction of unwanted sounds within the duct. That is, recently, special interest has been given to an electronic noise attenuation system of such active type in which noise propagated from a source of noise is sensed, a cancellation sound having the same sound pressure and an opposite phase with respect to the sensed noise is generated against the noise to provide sound wave interference between the noise and the cancellation sound, and thus the noise can be cancelled forcibly by the sound wave interference. With the rapid progress of an electronic device, signal processing technique and the like, there have been recently published various kinds of study results on such active electronic noise attenuation method and apparatus.
However, there are left many problems to be solved and thus such electronic noise attenuation method or apparatus has not yet come into a stage of seriously practical application.
A technical problem in putting into practice such electronic noise attenuation system consists in the construction of a model which can be used as a basis for design of a control system of the electronic noise attenuation system. The model must be able to cope with the following points. At first, there is necessary a filter which is capable of cancelling noise of continuous spectra. That is, if a cancellation sound can be generated with respect to the noise of continuous spectra such as automotive noise, air current noise and the like as well as the noise of discrete spectra such as transformer noise, compressor noise and the like, the applications of the electronic noise attenuation system can then be expanded further. To realize this, a filter is required which is able to provide arbitrary amplitude characteristics and phase characteristics.
Secondly, it is necessary to prevent the feedback of the cancellation sound with respect to a sensing microphone. That is, in the electronic noise attenuation system, there is interposed the sensing microphone between a source of noise and a source of cancellation sounds within a propagation passage through which sound waves are propagated, and it is necessary to create an electric signal to drive the cancellation sound source which generates sound waves to cancel the propagated sound waves from the noise source, in accordance with the sounds sensed by the sensing microphone and by some proper signal generation means. In this case, the sound waves generated from the cancellation sound source is also caught by the sensing microphone and, as a result of this, there is produced an acoustic feedback system between the cancellation sound source and the sensing microphone. For this reason, it is essential to take a countermeasure to cope with this situation. Especially, in order to make compact the electronic noise attenuation system and to allow it to be mounted at an arbitrary position in a pipe line such as a duct line, the sensing microphone and the cancellation sound source must be located adjacent to each other. Therefore, the above-mentioned acoustic feedback has a great influence on the electronic noise attenuation system and thus the counter-measure to cope with this problem is very important.
Thirdly, it is necessary to make it possible to correct the characteristics of electro-accoustic transducers such as a microphone, speaker and the like used in the electronic noise attenuation system. That is, in order to stabilize the the control function of the electronic noise attenuation system, it is essential that the control system of the electronic noise attenuation system is provided with a function to correct the minute amount of deterioration of the characteristics of the electro-acoustic transducers. This is another problem to be solved.
According to the conventional electronic noise attenuation systems of this kind, the above-mentioned technical problems have not been solved at all, making it impossible to put the conventional systems into practice.
In contrast with this, we have successfully come up with models on an electronic noise attenuation system using a monopole system as well as on an electronic noise attenuation system using a dipole system, both of which are able to cope with the above-mentioned technical problems as described in detail afterwards.
Out of our models, the model on the electronic noise attenuation system of the monopole type is able to perfectly deal with the above-mentioned first and third technical problems for realization of the electronic noise attenuation system. However, with regard to the second technical problem, that is, the prevention of the feedback of the cancellation sound with respect to the sensing microphone, since a control system for cancellation of such feedback becomes complicated in structure, the model cannot help employing passive means: for example, the consideration of the directivities of the respective electro-acoustic transducers such as the sensing microphone and the like as well as the positional relationships therebetween; and, the attachment of a sound absorption material to the inside of the propagation passage of sound waves extending from the cancellation sound source to the sensing microphone.
Also, the other model of our models mentioned above, namely, the electronic noise attenuation system of the dipole type according to the other model is able to cope with all of the above-mentioned three technical problems. However, it has been found too complicated in structure for practical application, although the control system thereof is simpler in structure when compared with that of the electronic noise attenuation system of the monopole type in realizing the prevention of the feedback of the cancellation sound with respect to the sensing microphone.
As described above, as the passive means to prevent the feedback of the cancellation sound, there are known several methods: in one of them, the directivities of mechano-electric transducing means such as a sensing microphone or electro-mechanical transducing means such as a speaker are improved for prevention of the cancellation sound feedback; and, in another method, the distance between the sensing microphone and the cancellation sound source is extended to reduce the energy of the cancellation sound.
However, in view of the fact that in the low frequency noises that produce a large amount of feedback the wavelengths are about several meters to several tens of meters, in order to provide the sensing microphone with an extreme directivity, the electronic noise attenuation system must be large in size whether it employs a waveguide or microphone arrays. This prevents the miniatuarization of a noise attenuator which is one of the effects given by the electronic noise attenuation system, making the system impractical. This is a common problem in a method in which the distance between the sensing microphone and the cancellation sound source is extended to prevent the feedback of the cancellation sound.
Further, in order to give a directivity to a speaker which is a typical example of the electro-mechanical transducers, there has been proposed a method in which three speakers are used to produce a directional sound source which generates only progressive waves. However, it requires a complicated control circuit but the effect thereof on the prevention of the feedback is not so great for the complication of the control circuit, that is, this method is disadvantageous in being not practical.
As discussed above, it is not easy to prevent the feedback of the cancellation sound with respect to the sensing microphone. But, it is currently requested that this problem is solved by a practical means.